Exam 3 - Gas Exchange and Transport Flashcards
partial pressure
defines diffusion of gas b/c allows comparisons of gas activities in both liquid and gas environments
Henry’s law
relationship b/w partial pressure and the amount of gas in physical solution
C = aP
C = concentration a = solubility P = partial pressure
Dalton’s law
Px = mole fraction of x (Fx)x total pressure (Pt)
-if the gas is a mixture, total pressure is the sum of the pressures exerted by each of the gases
talk about the partial pressure of water and its role in ventilation
PH2O in lungs = 47 mmHg -> lowers the partial pressure of the other gases in the lungs
describe the relationship between solubility and molecular weight
diffusion is inversely proportional to the square root of MW -> lighter gases achieve equilibrium faster
-for CO2 and O2, the diffusion ratio is 0.85, and in the gas phase, CO2 is 15% slower than O2
what determines the rate at which a gas diffuses through a liquid?
its MW + its solubility in the liquid
-CO2 (while slower than O2 in gas phase) is 20x faster than O2 in water (higher solubility)
Fick’s law of diffusion
V = [DA(P1-P2)]/X
D = diffusion coefficient A = surface area of membrane P1-P2 = partial pressure on either side of membrane X = membrane thickness
explain the pathophysiology of hyaline membrane disease
get thicker basement membranes -> low O2 saturation b/c less gas can move across
-fix by treatment with (+) pressure ventilators
what is membrane resistance?
1/Dm (physical) - resistance to diffusion imposed by the alveolar-capillary interface, plasma and erythrocyte membranes
what is chemical reaction resistance?
1/(theta x Vc) - resistance to diffusion imposed by the chemical reaction of O2 with Hb
theta = volume of O2 combining in 1 min w/ Hb in 1 mL of blood at partial pressure difference of 1 mmHg
Vc = pulmonary capillary blood volume
theta x Vc = rxn rate of O2 and Hb for the entire lung
what is diffusive resistance?
1/DL = membrane resistance (1/Dm) + chemical rxn resistance (1/(theta x Vc))
when Hb is saturated, what becomes the major factor in overall resistance?
theta - it depends on the % saturation of Hb, and when Hb is saturated, theta becomes small and chemical resistance will be the major factor in overall resistance
why is chemical resistance not important in O2 exchange?
b/c blood entering the lung, Hb is far from being saturated
why can chemical resistance be a factor in movement of CO2?
because reaction rate for CO2 release into the blood is much slower
how is it that exchange of respiratory gases is highly efficient even with these resistances?
the time for gas equilibrium to occur (250 ms) vs. the time required for the RBC to transit the pulmonary capillary (750 ms)
can exercise lower the RBC transit time to a dangerous rate? are there scenarios when it might?
nah - safety factor built into transit time in normal person
- if increased diffusive resistance, then end-capillary PO2 may not reach alveolar PO2 -> attenuated by exercise
- if high altitude (alveolar PO2 lower) -> increase in time for equilibration, can be a problem in exercise
what is diffusion capacity?
DL - the volume of gas moving per unit time for a given difference in partial pressure of a gas
DL = V/ (Palv - Pcap)
describe the physiological and pathological factors affecting diffusion capacity
- DL increases w/ body surface area
- DL increases w/ training
- DL decreases w/ thickening of alveolar-capillary membrane (pulmonary fibrosis, interstitial edema)
- DL decreases due to decrease in alveolar surface area (emphysema)
what are the normal blood gas values for:
- pH
- pressure O2
- saturation O2
- pressure CO2
- HCO3-
pH = 7.35 - 7.45 pressure O2 = 80 - 100 mmHg saturation O2 = 95-100% pressure CO2 = 35-45 mm Hg HCO3- = 22-26 meq/L
describe the pressure gradients to move O2 and CO2
O2: inspired air > alveoli > systemic arterial blood > tissues
CO2: tissues > systemic venous blood > alveoli > expired gas
what are the differences in the gas composition of inspired and alveolar air due to?
- contribution of anatomic dead space
- constant absorption of O2 and secretion of CO2
- humidification decreasing partial pressures of all gases
what percent of blood in the systemic arteries has bypassed the pulmonary capillaries?
2-3% - it therefore does not contribute to gas exchange
-includes Thebesian circulation (coronary veins draining directly to LV)
what is the result of some blood not contributing to gas exchange?
lower PO2 in systemic arteries (95 mmHg) relative to the PO2 in the alveoli (100 mmHg)
what is the ventilation-perfusion ratio?
ratio of alveolar ventilation Va to alveolar blood flow Qa
-average V/Q ratio is about 0.8
describe two extreme cases for ventilation-perfusion mismatch
- physiological shunt: when V/Q = 0 -> alveolus is unventilated
- physiological dead space: V/Q»_space;> 0 -> capillary is unperfused
what are the factors contributing to ventilation-perfusion mismatch?
- uneven ventilation (V): greatest at base, poorest at apex
- the smaller alveoli at lung base have higher compliance (deltaV/deltaP) due to surfactant effects and lower potential energy due to relaxed elastic elements -> for a given pressure, the smaller alveoli at base will expand more than those at apex
- uneven perfusion (Q): greatest at base, poorest at apex
- gravitational effects (max hydrostatic pressure at the base)
describe the changes in vascular and bronchiolar tone to compensate for ventilation-perfusion mismatch when V/Q»_space; 0 (in the case of physiologic dead space)
- bronchiolar constriction to reduce V
- vascular dilation to increase Q
describe the changes in vascular and bronchiolar tone to compensate for ventilation-perfusion mismatch when V/Q = 0 (in the case of physiologic shunt)
- bronchiolar dilation to increase V
- vascular constriction to reduce Q
what are the amounts of O2 and CO2 dissolved in blood?
O2 = 0.29 ml/dl - rest is carried by Hb CO2 = 25 times that due to higher solubility
describe the properties of Hb
- 64.5k MW
- makes up 90% of total cytoplasmic protein in the RBC
- 4 subunits (2 alpha + 2 beta)
- heme (porphyrin ring w/ Fe2+)
- prox His (F8) binds Fe2+ and distal His (E7) prevent apposition of second heme group
how much bound O2 is there in 100 ml of blood?
20.1 ml bound O2
describe O2 binding to Hb
cooperative - binding of one molecule of O2 facilitates the subsequent binding of other O2 molecules
describe the Hill plot
plot of log PO2 vs. log (theta/(1-theta))
- slope = Hill coefficient -> represents cooperativity
- shows Mb has no cooperativity, while Hb has positive cooperativity
what is the Bohr effect?
increased CO2 or H+ (acidity) shifts O2 dissociation curve to the right -> decreasing affinity of O2 for Hb
-so for a given PO2, less O2 is bound to Hb
what is the Haldane effect?
reverse of Bohr effect
increased O2 in pulmonary capillaries results in dissociation of H+ and CO2 from Hb -> left shift of O2 dissociation curve
what is the effect of BPG on O2 dissociation?
presence of BPG increases O2 affinity of fetal Hb to a value greater than that of material Hb
how does Mb respond to changes in pH, CO2, or BPG?
it is not affected by changes in these values - hyperbolic curve (no cooperativity)
tense T state vs. relaxed R state of Hb
deoxyHb = tense -> 8 salt links formed by carboxy terminal aa’s of each chain and their side-groups w/ groups on adjacent chains
oxygenation shifts alpha1 and beta2 subunits relative to one another, breaking strong salt links for weaker ones
oxyHb = relaxed
what mediates O2 cooperativity?
movement of the porphyrin iron atom: when O2 finds, it shifts Fe2+ into a planar position, breaking some of the salt bridges -> greater flexibility of subunits so their iron atoms can be more easily pulled into the porphyrin ring by O2
how does the amount of O2 delivered to tissues change based on changes in alveolar PO2?
the amount of O2 delivered to the tissues is unaffected by a moderate drop in alveolar PO2
-BUT patients w/ anemia: amount of O2 delivered falls dramatically as O2 saturation drops
what are some factors that cause a right shift in the O2 dissociation curve?
- increased H+ or lower pH (Bohr)
- increased CO2
- increased temp
- increased 2,3-BPG
(greater O2 delivery)
describe bicarbonate - removal of CO2 from tissues
CO2 + water –> H2CO3 (by CA) –> HCO3- + H+
-HCO3- transported out of RBC by exchanger for Cl- (this is the Cl- shift) -> causes net movement of water into the cell
this makes the hct of venous blood 3% greater than that of arterial blood
describe removal of CO2 from pulmonary capillaries
CA embedded in endothelium accessible to plasma -> HCO3- converted to CO2 and diffuses from plasma to alveoli -> breathed off
-oxygenation of Hb liberates H+, facilitating conversion of HCO3- into CO2
describe the CO2 dissociation curve/patterns
with deoxygenation in the tissues, Hb capacity for CO2 increases
with oxygenation in the lungs, Hb off-loads H+, causing production of CO2 at the pulmonary capillaries
what makes the pulmonary circulation a very low pressure, low resistance, high compliance system?
- difference in wall thickness b/w R (pulm) and L (systemic) ventricles
- arterioles of the pulm circulation have little smooth mm.
how does increase in pulmonary artery pressure (like with exercise) alter the PVR?
increase in pressure correlates with decreased PVR due to distension of these microvessels in the parenchyma tethered to the alveoli and recruitment of unperfused or poorly perfused capillaries due to their vasodilation upon alveolar inflation
